For many applications, it is necessary to convert a continuous analog signal into a digital signal so that the digital signal can be processed in the digital domain. One area of particular importance for this treatment is in the area of digital signal processing.
An analog-to-digital converter is a device which converts a continuous range of analog signals into digital signals. The major factors that determine the quality of an analog-to-digital converter are resolution, sampling rate, and speed.
Resolution in an analog-to-digital converter is the smallest change in voltage that an analog-to-digital converter can detect and that can produce a change in the digital signal. The resolution determines the total number of digital codes or quantization levels that the converter can produce.
Resolution of an analog-to-digital converter is usually specified in terms of bits of the digital code. An m bit code allows for 2.sup.m quantization levels of 2.sup.m -1 steps between quantization levels. Thus, a one-bit analog-to-digital converter has two quantization levels and will convert an analog signal into either a one and zero. A two bit code will convert an analog signal into a series of two bit digital code (i.e., 00, 01, 10, or 11) with a total of four quantization levels.
Analog-to-digital converters also have a maximum sampling rate that limits the speed at which they can perform conversions. The sampling rate is the number of times per second that the analog-to-digital converter can sample the analog signal. For proper preservation of information in a signal, the minimum sampling rate must be at least two times the highest frequency of the analog signal being sampled. The sampling rate of two times the highest frequency of the signal being sampled is called the Nyquist sampling rate. Nyquist converters are analog-to-digital converters that operate at the Nyquist rate. Oversampled converters are those that operate at multiples of the Nyquist frequency. Oversampled converters operate at ranges from 2 to 512, or even greater, times the Nyquist rate.
Conversion time is the time it takes an analog-to-digital converter to convert an analog signal into a digital signal.
Several different types of analog-to-digital converters exist. These include flash analog-to-digital converters, pipeline analog-to-digital converters, and successive approximation analog-to-digital converters. Another popular type is the delta-sigma analog-to-digital converter.
One drawback of delta-sigma modulators is that a slowly varying signal, a signal with a small amplitude, or a signal comprising a constant DC input may produce noise in the form of an audible tone. This noise is known as pattern noise or idle channel tones.